[clang] Add a -canonical-prefixes option

In https://reviews.llvm.org/D47480 I complained that there's no positive
form of this flag, so let's add one :)

https://gcc.gnu.org/PR29931 also has a pending patch to add the positive
form to gcc (but there's admittedly not a lot of movement on that bug).

This doesn't change any defaults.

Differential Revision: https://reviews.llvm.org/D108818